CANADA THISTLE, Cirsium
arvense (L.) Scop. 1, upper part of plant; 2, seed;
3, down with seed; 4, new shoot; 5, base of stem;
6, root system. Perennial, reproducing by seeds and horizontal
roots. Roots extend several feet deep and some distance horizontally.
Stems 2 to 5 feet (0.6 to 1.5 m) tall, grooved, branching only at
top, slightly hairy when young, increasingly hairy as they mature. Leaves
usually with crinkled edges and spiny margins, somewhat lobed, and smooth.
Flower heads numerous, compact, about 3/4 inch (1.9 cm) or
less in diameter, of lavender disk flowers only. Surrounded by bracts without
spiny tips. Male and female flowers usually in separate heads and borne
on different plants. Seed brown, smooth-coated, slightly tapered,
about 3/16 inch (4.5 mm) long and with a ridge around the blossom end.
Seed attached to tannish down that is easily broken off. Found in
all crops; a persistent and troublesome weed. While this description fits
most Canada thistles, there are a number of varieties differing slightly